The invention relates to an injection molding machine for plastic materials, the operation of which is governed by continuous monitoring of at least one measurement parameter, wherein there is provided a control which outputs a signal upon failure of a sensor which detects the measurement parameter.
In order to guarantee safeguarding of the apparatus and a minimum product quality, some parameters have to be continuously monitored in operation of an injection molding machine. In the event of a breakdown of the monitoring apparatus it is therefore necessary for the machine to be immediately shut down. Typically the stroke of the movable mold mounting plate and that of the ejector on the closing side and the stroke of the injection unit as well as the stroke of the metering screw in the measurement cylinder on the injection side are monitored. Limit switches monitor the attainment of predetermined positions, for example the end position of a nut. Temperature sensors are associated with the measurement cylinder, the cooling fluid for mold cooling and the hydraulic fluid which may possibly be provided. In the case of hydraulically actuated machines at any event the pressure in the injection cylinder and the pressure in each single hydraulic pump as well as the pressure in the high-pressure closing cylinder (pressure higher than . . . ) are monitored.
In many cases continuous monitoring but at any event automatic shut-down of the apparatus in the event of failure of the sensor is already not implemented for double pumps, in relation to closing cylinders which operate at low pressure and in relation to hydraulic storage devices. More specifically, in particular pressure and travel sensors exhibit the problem that their service life is difficult to predict and their operability terminates suddenly. The best procedure also in relation to temperature sensors is to observe a gradual worsening which indicates the need for replacement soon.
In itself the price of sensors is low in comparison with that of the machines. Replacement of the sensors is however a time-consuming operation as in particular travel sensors generally involve difficulties in terms of access and temperature sensors can only be replaced after the machine has cooled down. The attempt has therefore previously been made to make do with the minimum possible number of monitored measurement parameters, with only one sensor being associated with each parameter. The interruption in operation in the event of failure of the sensor has been tolerated.
The basis of the invention is the consideration that the expenditure not only for the provision of but also for the replacement of a relatively large number of sensors is justified if the number of unscheduled interruptions in operation can be reduced.
That object is attained in that at least two sensors are associated with one and the same measurement parameters and the control records the failure of a sensor without causing stoppage of the machine.
One and the same measurement parameter in accordance with the invention occurs when, in a situation involving ideal function, the associated sensors would display the same value. That is to be clearly distinguished from the situation where for example the stretch of each of the four beam members of an injection molding machine is individually measured and the closing pressure is calculated from the certainly different results. Even the fitment of strain gauges at different sides of a beam member in order to be able to detect flexing of the beam member would have nothing to do with the invention as the results supplied by the sensors would basically not be interchangeable.
The main advantage of the invention is that the replacement of failed sensors only has to be effected in the context of regular maintenance. As the sensors measuring identical parameters can be combined together in space, the increase in the number of sensors, in accordance with the invention, does not result in a proportional increase in costs. More specifically, two or more associated sensors can be combined together in a holder, while a housing which encloses the sensors can also contain the evaluation circuit which, besides the measurement value, also transmits the information about the state of the measuring sensing devices.
If the level of redundancy or the system is reduced due to failure of the sensors, that can be displayed to the exterior, which under some circumstances affords the possible option or restoring the normal condition while operation continues. That is the case in particular when it is provided that a shut-off device is provided between at least one sensor for monitoring a fluid pressure and the fluid being monitored.